The present invention relates to couplings in general, and more particularly to improvements in flexible shaft couplings. Still more particularly, the invention relates to improvements in flexible couplings which can be installed with advantage between the output shaft of a prime mover (e.g., an electric motor) and the shaft of a pump, e.g., the impeller shaft of a centrifugal pump which is used for circulation of liquids in a nuclear reactor plant.
Pumps which are used in nuclear reactor plants must be provided with readily accessible seals to insure that the seals can be rapidly inspected, repaired and/or replaced so as to avoid prolonged exposure of attendants to radioactive material which is likely to escape with leak fluid. The seals of such pumps are normally replaced at regular and rather frequent intervals. The periods of time which are necessary for replacement of defective or potentially defective seals are reduced by installing a readily detachable and relatively short intermediate shaft between the pump shaft and the motor shaft so that a seal which surrounds the pump shaft can be reached and replaced upon detachment of the intermediate shaft from the other two shafts. If the pump shaft is provided with a flange, the flange is normally separable from the remaining portion or shank of the pump shaft so that the seal can be slipped off the shank.
In order to insure that the shaft seal or seals of a pump which is used for circulation of contaminated fluids in a nuclear reactor plant will stand relatively long periods of use, the pump shaft as well as the parts which drive the pump shaft must be machined and mounted with a high degree of precision. However, even the most accurate machining cannot prevent at least some axial, radial and/or angular misalignment of components of the torque-transmitting connection between the prime mover and the impeller of the pump. It has been found that at least some misalignment (especially radial and/or angular misalignment) is unavoidable in motor-pump aggregates of the above outlined character, not only as a result of machining tolerances but also owing to wear upon the moving parts, changes in temperature and/or other unpredictable factors.
The situation is aggravated if one or more rotary members of the torque transmitting connection between a prime mover and the impeller of a pump are long and heavy enough to necessitate mounting in two or more axially spaced friction and/or antifriction bearings. Even minute misalignment of neighboring bearings will result in very pronounced wear upon the bearings and/or shaft seals, especially since the shafts which are mounted in multiple bearings are normally extremely heavy, bulky and hence highly likely to deform or destroy misaligned bearings and/or shaft seals after very short periods of use. The magnitude of stresses which a shaft transmits to multiple bearings and shaft seals depends on the extent of misalignment of the bearings. Misalignment of two or more bearings for one and the same shaft can be reduced by resorting to highly expensive aligning equipment and by further reducing the machining tolerances. This can be achieved by employing skilled persons and by frequent inspection of the shafts, their bearings and/or seals. However, all such measures cannot prevent misalignment of plural bearings for a relatively long, heavy and bulky shaft if the temperature of conveyed fluids and/or the temperature of the surrounding atmosphere fluctuates within a wide range. Thus, the bearings will be accurately aligned when the shaft and/or the bearings are cold but the shaft will run out of true as soon as the temperature rises, especially if the change in temperature is not uniform from bearing to bearing.
Heretofore known proposals to reduce the detrimental effects of such misalignment of bearings and the resulting rapid wear upon the seals for the shaft of a pump include the utilization of flexible shaft couplings which compensate for eventual axial, angular and minor radial misalignment of neighboring rotary members. However, conventional flexible shaft couplings are not suited for use in pumps which are employed in nuclear reactor plants because they cannot stand the corrosive influence of circulated fluids and also because they must be lubricated at frequent intervals (lubricant is undesirable because it is likely to contaminate the circulated fluids as well as because it is likely to escape and to entrain radioactive material into the surrounding area where the radioactive material can affect the health of attendants.
In nearly all presently known flexible shaft couplings, the elastically deformable part is a washer which is made of rubber or elastomeric synthetic plastic material. The useful life of such elastic components is very short and, furthermore, they cannot be used in aggregates wherein a prime mover transmits substantial torque and/or substantial axial stresses to the shaft of a centrifugal pump or the like. Therefore, the just described flexible shaft couplings can be used only when the transmitted torque and/or axial stress is relatively small.
German Pat. No. 176,486 discloses a flexible shaft coupling which need not utilize any components that are made of rubber or the like. The coupling is intended to transmit substantial torque as well as to take up pronounced axial stresses. To this end, the flange of one of the shafts has a concave internal surface which surrounds a complementary convex external surface on the respective shaft and the screws or bolts which secure the just described flange with the flange of the other shaft extend through spherical bushings. The abutting concave and convex surfaces, as well as the surfaces of the bushings, must be lubricated at frequent intervals in order to avoid premature wear and overheating as a result of frictional engagement between the abutting surfaces. As a rule, an apparatus or a machine (e.g., a vehicle) which utilizes the patented flexible coupling must be provided with a system which automatically feeds lubricant to the coupling whenever the apparatus or machine is in use. Therefore, such flexible coupling cannot be used in the torque transmitting connection between a prime mover and the impeller of a pump which is used for circulation or conveying of fluids in a nuclear reactor plant. As mentioned above, lubricant which is supplied to movable parts on a continuous basis is bound to escape and to entrain at least some radioactive material. Furthermore, some lubricant is also likely to come in contact with the conveyed fluid. Finally, the patented coupling requires continuous and pronounced cooling, especially if it is to transmit substantial torque and/or to take up pronounced axial stresses. Continuous cooling entails the circulation of a fluid which is likely to be contaminated by radioactive material and/or to come into contact with water or another liquid which is circulated by the pump.